After you have masturbated for a good, leisurely long time (with many stops and starts!), set some kind of timer, alarm clock, or digital watch to go off in 10 minutes – and don't let yourself ejaculate until the timer goes off. You should also have a digital clock in the room to note the time. (This can be the same clock as the one with the alarm, as long as it doesn't display seconds – you want to know the time to the minute, but not down to the second.) Refrain from looking at the clock for the first few minutes. As the ejaculation time draws near, occasionally glance at it – but don't stare. When there's one minute left on the clock, you'll go crazy with anticipation because the alarm could ring at any second. Try to hover right on the edge so you can orgasm immediately when the alarm rings. It will seem like an eternity!
It's especially good if the alarm has a fairly long ringing cycle. If it continues to ring as you begin ejaculating, it will heighten the sense of urgency and tension relief.